We have made experimental studies of low-temperature, high-magnetic field electronic transport in layered, three-dimensional semiconductor structures. In fields that produce the integer quantum Hall effect for transport parallel to the layers, we find deep minima in the vertical conductance $G_{\mathrm{zz}}$. Between these quantum Hall states, the size dependence of $G_{\mathrm{zz}}$ indicates that vertical transport is through the bulk. Within quantum Hall states, we find that vertical transport is along the surface of the sample, via a “sheath” of extended surface states. This surface sheath is a novel two-dimensional system that exhibits a metallic conductivity $\ll e^2/h$.

• Received 5 June 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.365